A violet semiconductor laser, made of Group III-V nitride semiconductors (AlxGayIn1-x-yN (where 0≦x≦1 and 0≦y≦1) such as gallium nitride (GaN), is a key device for realizing ultrahigh density recording by optical disk drives, and is about to be actually used in consumer electronics products. The output of a violet semiconductor laser must be increased not just to enable high-speed writing on an optical disk but also to develop new fields of technology including application to laser displays. A conventional violet semiconductor laser is disclosed in Non-Patent Document No. 1, for example.
Recently, a GaN wafer has been regarded as a strong candidate for a wafer for fabricating a nitride-based semiconductor device thereon. This is because a GaN wafer is superior in the degree of crystal lattice matching and heat dissipation ability to a sapphire wafer that has been used in the pertinent art. Another advantage is that the GaN wafer has electrical conductivity, whereas the sapphire wafer is an insulator. That is to say, a structure in which current also flows across a GaN substrate can be adopted by arranging an additional electrode on the back surface of the GaN substrate, too. If an electrode is arranged on the back surface of a GaN substrate with electrical conductivity, then the size (i.e., the chip area) of each semiconductor device can be reduced, thus increasing the number of chips that can be made out of a single wafer. As a result, the manufacturing cost can be reduced.
A GaN wafer may be made in the following manner, for example. First, a GaN single-layer film is grown on a sapphire wafer by an MOVPE process. Thereafter, a thick GaN film is grown on the GaN single-layer film by a hydride VPE (HVPE) process, for example. And then the sapphire wafer is removed.
A GaN wafer obtained in this manner has dislocations (including edge dislocations, spiral dislocations and mixed dislocations) at a density of about 5×107 cm−2. If the dislocation density is that high, it is difficult to make highly reliable semiconductor lasers. In addition, the uppermost surface of a GaN wafer made by an HVPE or any other process may have pits, hillocks and so on, and therefore, may sometimes have an unevenness of about 0.1 mm. Such unevenness on the principal surface of a GaN wafer will pose a big obstacle to a photolithographic process, for example, thus decreasing the production yield of devices.
To iron out such unevenness on the principal surface of a wafer, the principal surface of a wafer needs to be polished and planarized. Since GaN is highly resistant to chemicals, it is difficult to planarize the GaN wafer by chemical polishing, and therefore, mechanical polishing is usually adopted. In that case, however, the surface of the GaN wafer often gets scratched and damages are often left in the vicinity of the surfaces of crystals.
Besides, machining strains (residual strains) are often left on the surface of the wafer and have an in-plane distribution in many cases. When observed using an atomic force microscope (AFM), the scratched had a depth of several tens of μm and the root mean square (RMS), calculated on an area of 50 μm square, was 1.6 nm. If GaN crystals were grown as they are on the principal surface of such a GaN wafer, then the surfaces of crystals would be seriously affected by those scratches.
To make the density of dislocations in the nitride-based semiconductor layer that has been grown on a GaN wafer lower than that of the GaN wafer, an epitaxial lateral overgrowth (ELO) has been adopted. Hereinafter, the epitaxial lateral overgrowth will be described with reference to FIGS. 12(a) through 12(d).
First, as shown in FIG. 12(a), a GaN wafer 1001 is provided and a mask layer 1003 of SiO2 is formed thereon. The mask layer 1003 has striped openings that selectively expose regions of the principal surface of the wafer that will function as a seed for crystal growth.
Next, as shown in FIG. 12(b), an epitaxial lateral overgrowth process is carried out as an MOVPE process, thereby growing an n-GaN layer 1002 from the respective openings of the mask layer 1003. In this process step, conditions for growing GaN crystals less easily on the mask layer 1003 are adopted. However, polycrystalline GaN sometimes precipitates on the mask layer 1003, too. The GaN wafer 1001 often has n-type conductivity. Thus, by supplying not only a source gas of gallium nitride but also monosilane (SiH4) and disilane (Si2H6) onto the GaN wafer, a GaN layer 1002 having n-type conductivity is formed.
If the n-GaN layer 1002 continues to be grown as shown in FIG. 12(c), then adjacent portions of the n-GaN layer 1002 will soon be combined with each other, thus forming a single continuous layer as shown in FIG. 12(d).
The n-GaN layer 1002 formed by such a process has regions where the density of dislocations is reduced to 7×105 cm−2 or less. If the device structure is fabricated over such regions with a reduced density of dislocations, then the reliability can be increased. However, if polycrystalline GaN precipitates on the mask layer 1003 as shown in FIG. 12(b), then crystallinity deteriorated regions 1004 will be produced as shown in FIG. 12(c).
To further reduce the density of dislocations, Patent Document No. 1 discloses a semiconductor device in which a mask layer is formed in striped recesses and an air gap is provided over the mask layer. FIG. 13 illustrates a structure including an n-GaN wafer 101 in which recesses are covered with a mask layer 103 and an n-GaN layer 103 that has grown from striped ridges. The n-GaN layer 103 includes low-dislocation regions 104 with a relatively low density of dislocations and high-dislocation regions 105 with a relatively high density of dislocations. A ridge stripe 106, defining a current injection region, for example, is arranged on a low-dislocation region 104 on the n-GaN layer 102.                Non-Patent Document No. 1: Japanese Journal of Applied Physics (Jpn. J. Appl. Phys.), Vol. 39. p. L647, 2000        Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 2002-9004        